1. Field of the Invention
The present invention relates to an image forming apparatus provided with plural color developing units positioned opposite to an electrophotographic photosensitive member, and adapted to transfer images, composed of developing agents and formed in succession by said developing units on the photosensitive member, onto a transfer member.
2. Related Background Art
FIG. 4 illustrates a conventional electrophotographic color image forming apparatus for forming a full-color image, in which a photosensitive drum 1 serving as the photosensitive member is supported rotatably in a direction R1, and a charging device 2, an optical system 3, a developing device 5, a transfer device 6 and a cleaning device 7 are positioned around said photosensitive drum 1.
The optical system 3 is composed of a laser beam exposure device, provided with an original scanning unit and color separation filters and adapted to irradiate the photosensitive drum 1 with a color-separated light image E or a light image corresponding thereto. The photosensitive drum 1, uniformly charged by the charging device 2, is subjected to scanning irradiation of the light image E for each separated color, thereby forming an electrostatic latent image. The developing device 5 is constructed as a rotatable developing device composed of a rotary member 5a having a center of rotation at a central shaft 5b, and four color developing devices, i.e., a yellow developing device 5Y, a magenta developing device 5M, a cyan developing device 5C and a black developing device 5BK, and is adapted to move, by rotation, a desired developing device to a developing position opposed to the photosensitive drum 1 and to effect reversal development of the electrostatic latent image on the photosensitive drum 1, thereby forming a toner image composed of electrophotographic color toner on said photosensitive drum 1.
The toner image on the photosensitive drum 1 is then transferred onto a recording material P by the transfer device 6, which is provided with a transfer charger 6b, an attraction corona charger 6c for electrostatically attracting the recording material P, an attraction roller 6g positioned opposite thereto, an internal corona charger 6d and an external corona charger 6e, and, on a peripheral aperture of a rotatably supported transfer drum 6a, a recording material support sheet 6f, consisting of a dielectric material, is integrally extended in cylindrical form. The recording material P is supplied to the transfer device 6 from a cassette 9 through a transport system.
With the rotation of the transfer drum 6a in a direction R6, the toner image on the photosensitive drum 1 is electrostatically transferred, by a polarity of the transfer charger 6b opposite to the polarity of uniform charging of the photosensitive drum 1, onto the recording material P supported on the support sheet 6f. Thus, color images of a desired number are transferred in succession onto the recording material P supported by attraction on the support sheet 6f, thereby forming a full-color image.
After transfer of the desired number of toner images, the recording material P is separated from the transfer drum by a separating charger 6h and separation means 10, then is subjected to fixation of the toner images by a fixing device 11 provided with heated rollers, and is then discharged onto a sheet discharge tray 12.
On the other hand, after the toner image transfer, the photosensitive drum 1 is subjected to a cleaning operation where toner remaining on the surface is cleaned off by cleaning device 7, and is subjected again to an image forming process.
However, the above-explained conventional technology has been associated with the following drawbacks. As the polarity of toner charging is opposite to that of uniform charging of the photosensitive drum 1 (for example the photosensitive drum 1 is charged negatively while the transfer charging is charged positively), the charge of the transfer charging induces a transfer memory on the photosensitive drum 1 through the recording material support sheet 6f and the recording material P, whereby uniform charging of the photosensitive drum in the next image forming process may become uneven and, in case of reversal development, the final image density may become uneven.
Particularly when the photosensitive drum 1 is small in radius so that the peripheral length thereof is shorter than the maximum length of the recording material, uniform charging in the first turn of the photosensitive drum 1 is not affected by the image transfer but uniform charging in the second and subsequent turns is influenced by the transfer memory effect, thereby generating so-called density step, namely a phenomenon of a higher density in the image formed in such second and subsequent turns. Such transfer memory effect is particularly conspicuous when an organic photoconductor, advantageous in safety and cost, is employed on the photosensitive drum 1.
As a countermeasure for such phenomenon, there has been conceived an apparatus provided with an additional charger for effecting a charging step after the image transfer and prior to the uniform charging, in order to eliminate the transfer memory, but the influence of image transfer may still appear as a step difference in density, as it is difficult to eliminate such influence of transfer, following the variations for example in the moisture absorbing characteristics of the recording material P, in the electrostatic capacitance of the photosensitive member, etc.
Also in the conventional technology explained above, the developing device 5 of the rotary type effects reversal development of the electrostatic latent image on the photosensitive drum 1, by rotating the developing device of a desired color to the developing position opposed to said photosensitive drum. For this reason it is necessary to secure a time required for the rotational mechanical movement, and, in an image forming mode supporting plural recording materials P on the transfer drum 6a, the developing devices cannot be exchanged within the time between the recording materials. The transfer drum 6a has to be rotated unnecessarily in order to secure such exchanging time, so that the overall copying speed is inevitably lowered significantly.